Elevators may be driven by induction motors.
By means of vector control of induction motors, the primary current is divided into exciting current and torque current and the vector of the torque current and flux are crossed in order to obtain the same speed variation capability as with direct-current motors or better.
FIG. 3 is a diagram of one example of a vector control device. The numeral 1 in the figure is an induction motor and 2 is a speed detector. A speed computing section 3 is connected to the speed detector 2. Speed W.sub.n that is computed from the detected values by the speed computing section 3 and speed command N are balanced in a summer 3a and the difference into a torque computing section 4. A torque current command (I.sub.t) is determined by a proportional integration (PI) computation. A primary current value (I.sub.1) is determined from this torque current (I.sub.t) and an exciting current set value (I.sub.o) by primary current computing section 5 using I.sub.1 =.sqroot.I.sub.t.sup.2 +I.sub.o.sup.2. On the other hand, the phase angle .phi. of the torque current (I.sub.t) and exciting current (I.sub.o) is computed by phase computing section 6 using, .phi.=tan.sup.-1 (I.sub.t /I.sub.o). Slip frequency computing section 7 determines the slip frequency (W.sub.s) from torque current (I.sub.t), exciting current (I.sub.o), and secondary time constant .tau..sub.2 of the motor as shown below; furthermore, time constant .gamma.2=L2/R2 for secondary auto-inductance L2 and secondary resistance R2. EQU W.sub.2 =I.sub.t (I.sub.o .multidot..tau..sub.2)
This slip frequency (W.sub.s) is added to the speed detected value (W.sub.n) by adding device 8 to obtain the primary frequency (W.sub.o).
The aforementioned primary current (I.sub.1), phase angle .phi. and angular frequency (W.sub.o) are input to a three-phase current computing section 9 to calculate primary currents I.sub.a, I.sub.b, and I.sub.c of motor 1. Using this current as the current command of inverter 10, primary current is fed to motor 1 by inverter 10. Computation by aforementioned three-phase current computing section 9 is performed by the following equations: ##EQU1##
By means of this type of vector control device, secondary time constant .tau..sub.2 for determining slip frequency W.sub.s is determined by constants L2 and R2 of the motor and changes in secondary resistance R2 with temperature of the motor are regarded as changes in secondary time constant .tau..sub.2 and the primary current phase. Therefore, secondary time constant .tau..sub.2 is corrected for temperature by computation from the primary voltage when a constant current is passing to the motor. FIG. 4 shows a graph of changes in primary voltage V.sub.1 (t) when constant current i.sub.l is flowing. Primary voltage V.sub.1 (t) is approximated by
Therefore, secondary time constant .tau..sub.2 is found with the following equation by determining primary voltages V.sub.1 (t.sub.1) and V.sub.1 (t.sub.1) at times t.sub.1 and t.sub.2. ##EQU2##
The effects of temperature can be eliminated by using this secondary time constant .tau..sub.2 in computations of slip frequency.
In the aforementioned conventional method for correction of the secondary time constant, the steady-state values of i.sub.1 and r.sub.1 at primary voltage V.sub.1 (t) are needed for determination of the secondary time constant and it takes a long time until primary voltage V.sub.1 (t) reaches the steady-state level in these determinations. Consequently, there is a problem in that it takes a long time to prepare for operation of the motor and a control device is obtained whose use is off to a bad start.